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**A feature flag** is a mechanism that lets you **turn certain application features on or off without changing the code and without a new release**.
It gives the team control over **who sees new functionality and when**.

### How it works

- In the code, the new feature is wrapped in a condition:

```python
  if feature_flag("new_ui"):
      show_new_interface()
  else:
      show_old_interface()
  ```
- The flag is stored in configuration (a file, a database, or a service like LaunchDarkly, Unleash).
- Developers or product managers can **turn on the feature selectively** - for example, for only 10% of users or a specific group.

### Why feature flags are needed

1. **Safe rollout**
A new feature can be enabled gradually (rollout), while tracking metrics and errors.
2. **A/B testing**
One flag, different interface variants for different users.
3. **Fast shutdown of problematic features**
If a new feature causes bugs, it can be turned off instantly without rolling back the release.
4. **Release flexibility**
Development and deployment are decoupled: the code can be in production but "asleep" until the flag is turned on.

### Usage examples

- A "dark theme" is enabled only for internal testers.
- A new recommendation algorithm is enabled for 20% of users.
- After a release, a failed feature is turned off with one click.

### Conclusion

**A feature flag** is a "switch" inside the product that lets you manage functionality **without new releases or risk**.
It makes the system more flexible, safer, and better suited for **Continuous Delivery**.

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